A wide-area scanning tunneling microscope configured for metallurgical research

Abstract:

This thesis describes research into the design of a scanning tunneling microscope (STM) intended for investigating the structure-dependent resistivity of noble metal alloys. A survey of existing designs showed that there was a wide variation in methods and that in many respects there was no consensus regarding an optimum technique. A number of elements of the STM were singled out as offering considerable scope for improvement; these included the coarse approach mechanism, the feedback control system, the implementation of an XY stage, and the implementation of scanning tunneling potentiometry (which was a necessity in the finished instrument). These elements of design were investigated with the intention of evaluating alternative methods for inclusion in a final design. Whilst the final design was purpose-specific, many of the evaluation results have considerable applicability to the field as a whole. The mechanical structure of an STM is to a large extent determined by its coarse approach mechanism. Reduction mechanisms based on cantilevers were thought to be promising, and did not appear to have been widely used. A differential reduction system using two cantilevers, and a simple single-cantilever reduction system were constructed. It was found that a single cantilever, used as a 10 : 1 reduction mechanism for a screw drive, offered high rigidity and simplicity of manufacture; a successful STM based on a cantilever was constructed. The implementation of control in the cantilever STM raised questions about the sources of resonant behaviour in the STM actuator; it was established that these had not been studied in any detail. As this is a primary factor in feedback control, a comprehensive study of actuator resonance was performed. This study necessitated the development of wide-bandwidth techniques for measuring small displacements, some of which are described. The study revealed that the method by which the actuator is mounted can affect the resonant behaviour, but that with sufficiently rigid mounting the lowest resonance is likely to be the first natural off-axis mode in a tube actuator. This study also showed that if the tube is rigidly mounted, a second-order (resonant) model is more accurate than a first-order (delay) model for the tube for control purposes; but that neither model accurately reflects the phase behaviour at resonance. This more accurate model allowed examination of two unresolved issues in feedback control of STMs; the problem of "flying" (losing contact on downhill slopes) and the optimisation of digital controllers. It was found that "flying" could be solved by introduction of an extra integral element, but that this was unlikely to represent a good compromise of speed and accuracy for most users. The stability of digital integrators was examined; and it was shown that limit cycle output oscillations would almost inevitably occur with conventional digital controllers. A set of mathematical conditions for numerical and quantisation accuracy were derived which if met would prevent the limit cycle behaviour. A novel digital controller was constructed which complied with these conditions and was found to work satisfactorily. An offshoot of this work into characterising actuators was the development of a dual-actuator STM which allowed the extension of control to much higher frequencies than have previously been achieved. This work has provided some insight into the eventual limits of STM speed. The inertial-slider principle was used in designing a novel XY stage that offered two-dimensional translation from a single piezoactuator. The stage, which required no mechanical connection, could be moved rapidly (in steps as small as 20 nm) over an area of 9 mmx 9 mm. Conventional scanning tunneling potentiometry systems can only operate with a one-dimensional potential variation, and are generally only used on specimens of relatively low conductance (such as semiconductors or thin metallic films). A novel system for two-dimensional scanning tunneling potentiometry (STP) was implemented. Synchronous detection at moderately high frequencies was used to obtain topographic, and X and Y potential information without interruption of feedback or the introduction of excessive noise; the high frequency, low noise technique allowing STP imaging of samples with much lower resistances than had previously been imaged. All of these investigations explored areas which had not previously been examined by STM researchers, and the results obtained go some way to indicating the future direction of progress in those areas; the control results, in particular, are relevant to a large proportion of work in the SPM (scanning probe microscope) field.